A goal with speaker systems is to faithfully reproduce a sound. This sound may be from a recording, or directly from a real-time audio source, such as a microphone. A simple speaker (also referred to as a driver) hanging in free air is a poor reproducer of sound—sound waves from the back side of the speaker interfere with the waves coming forward. Each speaker has frequency response characteristics which make it good in one frequency range and poor in another. For example, a twelve inch woofer is designed to be able to reproduce low frequency sounds well, but the speaker cone has such large mass that it does a relatively poor job of reproducing higher frequencies.
One technique of improving the sound from a speaker is well known in the art: eliminating the sound waves from the rear of the speaker cone by putting the speaker into a wall. The wall serves as a baffle preventing the sound waves from the backside from emerging and interfering with the sound weaves coming off the front of the speaker. That not always being convenient, it was found that putting a speaker into a sealed box had the same effect, hence a sealed box speaker also being referred to as an infinite baffle enclosure. A variety of configurations have emerged for speaker enclosures of this type.
Another technique developed of combining two or more speakers in a single enclosure, or in separate enclosures but reproducing essentially the same sound source. For example, a large cone speaker (woofer) might be combined with a smaller hard cone high frequency speaker (tweeter) to get better overall sound coverage. It was quickly noted that putting low frequencies into the tweeter resulted in poor sound reproduction, and so the frequencies were split using various designs of crossover networks to shunt higher frequencies to the tweeter and lower frequencies to the woofer. A variety of passive and active crossover networks are well known in the art.
Yet another improvement can be obtained by putting a vent in the front of the speaker enclosure. By properly sizing the vent and enclosure, the frequency response in the low range of a woofer can be improve substantially over an un-vented enclosure, generally obtaining a one octave improvement. This results in flat frequency response curve over a broader and lower frequency range than a closed box will provide, in effect providing a resonant chamber to assist the speaker in producing low frequency sound. While the roll-off rate at the low end of the frequency response is sharper, it occurs at a lower frequency.
Choosing port dimensions was, until the 1970s, a trial-and-error affair. Neville Thiele and Richard Small devised an analysis method that could predict the frequency response performance, and other characteristics of a loudspeaker system and enclosure, based on the physical and electronic properties of the devices used and the enclosure designed. Most loudspeaker manufacturers now provide these characteristics as part of the speaker specifications known as Thiele-Small parameters from which ideal (i.e. getting the best desired performance) enclosures for that device may be designed and built. Of course, any design ends up being something of a compromise between optimum sound reproduction and the practical physical size limitations of an enclosure. Often, quality is sacrificed for reduced size.
Because speaker enclosures have in recent times been designed using the Thiele-Small parameters, the resultant enclosure has a predictable internal volume, relative to the size and desired performance of the selected drivers. There is typically a box with a top, bottom, sides (which need not be parallel), front and back. (In terms of the physics of the enclosure, it makes no difference which is considered the front, back, sides or top. This is because it is primarily the volume of air in the box that is most important to the functioning of the box and drivers. It is usual to choose the largest physical surface as the front, or baffle.)
The front baffle of any speaker enclosure is equally, if not sometimes more, important that the enclosure. This is because the speaker (or driver) which is required to produce the low frequency components of the sound source is typically mounted directly on the baffle.
In many cases, the low frequency driver is placed on a surface (the front baffle) of inadequate physical area to propagate the low frequency sound waves to their full potential. Instead, the designers go to great lengths to manipulate the enclosure/port dimensions and the electronic crossover network to achieve the best (i.e. desired compromise) and most appropriate acoustic performance for the selected drivers. In many cases, certain performance results are achieved at the expense of others.
The best speaker systems still have a limited dispersion angle left and right of center in front of the speaker angle at which a constant sound pressure level is produced, ideally the same for the desired range of frequencies to be reproduced. If a listener is too far off axis, the sound was not satisfactory, thus placing limitations on where in a room speakers could be placed and where the listener should be relative to the speakers in order to achieve maximum fidelity.
A room can have substantial effects on the reproduction of sound, depending on the number and position of reflecting surfaces and diffracting edges that can interfere with the sound as initially produced at the speaker cone.)
Another perceived deficiency of many speaker systems is harder to articulate or quantify, but nevertheless quite real. It is best described as the “lack of dimensionality” of the sound produced. Simply put, some speaker systems seem to give a greater impression of the sound space in which the sound being reproduced was generated. The sound produced seems subjectively more three-dimensional, although this effect often is experienced only in certain spots in front of a particular speaker system.